Infrared detectors have been used to control lights and other electrical appliances. Such devices detect the change in the infrared radiation (heat) level within an area and activate the electrical appliance or sound an intrusion alarm. Typically, the change in heat results from a person entering or moving within the sensing area. The appliance remains turned on for a predetermined period of time after which, if no further change in the infrared level has occurred, the appliance goes off.
It is desirable that such devices be sensitive to relatively small changes in infrared radiation. These devices may employ a window discriminator which produces an output either when the detected radiation exceeds an upper threshold or falls below a lower threshold. As shown in U.S. Pat. No. 4,179,691, this discriminator may comprise two comparators. One comparator has a reference voltage applied to its inverting input and the other comparator has a lower reference voltage applied to its noninverting input. The reference voltages may be supplied by a single voltage divider. The other input of each comparator is connected to the output from the infrared detector.
The sensitivity of the window discriminator and hence the entire device is dependent upon the shortness of the window or in other words the difference between the two reference voltages. The typical discriminator described above has a practical limitation on how close these voltages can be set. The tolerances of the resistors in the voltage divider may cause an overall upward or downward shift in the window. Also the voltage input from the detector may vary due to tolerances in its circuitry. Therefore, the window must be tall enough to tolerate these voltage variations due to differences in the circuit components.